Recording/playback apparatuses using optical recording media such as compact disks (CDs) and digital versatile disks (DVDs) are known. Information-writable optical disks such as CD-R/RW, DVD+RW, and DVR have address information recorded thereon by wobbling guide grooves (in other words, address information recorded by optical modulation based on an address signal is embedded as groove wobbles).
FIG. 9 schematically shows wobbling on a disk, where wave-shaped lands and grooves are arranged alternately in the radial direction of the disk.
For DVD-R/RW, land prepits (LPP) represent address information, as shown in FIG. 10.
Guide groove wobbling is seen in both formats where information detected as wobbles is used for disk rotation servo (CLV servo) control, phase-locked loop (PLL) frequency locking of recorded clocks, and speed component detection, etc.
To acquire address information, a push-pull (PP) signal is used during wobble detection.
“Push-pull signal” here designates a light-difference signal (a so-called radial push-pull signal) detected by a split detector. In detail, as shown in FIG. 11, light radiated onto an optical disk is reflected at the recording layer of the disk, and a spot S of the returning light is received on a two-split detector PD. The detector PD is divided into two areas by a split line a running through its center. The difference between the signal received at one area (area A) and the signal received at the other area (area B) is calculated by a subtractor b, whereby the radio frequency (RF) components in the signal are eliminated (elimination of common-mode components) to produce a push-pull signal.
A signal containing address information (address signal) is more difficult to detect after data is written onto a disk, in which case disturbance from pits (record marks) affects the detection, than before the data is written onto the disk.
A known structure has the following problem with address detection based on a push-pull signal (with reference to FIGS. 12A to 12C).
The elimination of common-mode components described above assumes that the spot S of returning light lies just at the center of the two-split detector PD (with reference to FIG. 12B). For this reason, elimination of common-mode components is not satisfactory in the presence of a shift of the light spot towards either side from the center of the two-split detector, such as when the spot S lies biased towards the area B (as shown in FIG. 12A) or the area A (as shown in FIG. 12C) of the detector.
If a shift in an objective lens due to, for example, deflection causes the light spot on the two-split detector to be positionally deviated, signals of respective amounts of light become unbalanced, preventing satisfactory elimination of RF components from the signals. As an unfortunate result, it becomes difficult to acquire recorded address information or the structure of detection circuits becomes complicated.
Accordingly, an object of the present invention is to ensure that information recorded on an optical recording medium (including address information) is easily played back.